1. Field of the Invention
The invention relates to digital character generation as applied to random access display systems. The invention is of particular use in random access radar cathode-ray tube displays although it is by no means limited thereto.
2. Description of the Prior Art
The earliest display systems wherein it was desired to synthetically produce a random assortment of characters and symbols and to display them on a cathode-ray tube display screen either by themselves or interspersed with other data presentations was the raster scan display technique. In this type of display characters were typically generated as a pattern of dots, the dots lying along the raster scan lines. The dot patterns were usually stored digitally as patterns of binary numbers. Although such systems were relatively inexpensive, the quality of symbol produced was limited by the density of raster scan lines. Moreover, one dot had to be stored for each possible dot location in the patterns. With such systems, it was difficult to produce complicated character patterns because of storage requirements and available raster line density.
The advent of random access display systems wherein the beam may be positioned to any addressed point on the face of the cathode-ray tube made it possible to generate character or symbol patterns as continuous cursively written patterns much as one would write such a pattern by hand. The elimination of the requirement that the character or symbol patterns be produced with dots made it possible to produce patterns of widely varying sizes while yet retaining the readability of the patterns. Moreover, the patterns were then not restricted to what could be produced by dot character and symbol generators.
In early cursive character and symbol generators, the patterns were generated by using analog techniques much as is used to produce the familiar Lissajous patterns. Of course, in such systems the variety of patterns was limited by the available analog waveform generating techniques.
The next group of character and symbol generator approaches generated the character or symbol patterns by a series of interconnected strokes. The strokes were produced by interrelated X and Y ramp shaped signals. The ramps were produced by activation of selected resistors in respective X and Y resistor weighing networks. Such systems were capable of producing good quality cursive character and symbol patterns but they tended to be relatively expensive because of tight tolerance requirements upon the resistance value of the weighing resistors, the number of resistors required, and the relative complexity of the control circuitry. Many adjustments were required to compensate for skew delaying between X and Y channels, the gain of the two channels, and for the relative frequency responses of the two channels. The character patterns were frequently subject to distortion by noise present in the DC power supplied to the system as well as to any noise present within the circuitry itself such as cross talk between the X and Y channels.
Attempts to alleviate the problems inherent in the previously described system included one in which the character or symbols were generated by stroke patterns and in which the end points of the stroke patterns were stored in a read only memory. The patterns were generated by producing signals representing the interconnection of the end points. The expensive resistor weighing networks were thereby eliminated. However, the system was still subject to noise because of the difficulty of producing analog circuitry for generating the signals connecting the prestored end points. Moreover, additional difficulties were encountered in producing end point connecting signals wherein the rate of movement of the beam upon the cathode-ray tube screen was substantially constant among all strokes within all the character or symbol patterns within the stored repertoire. Changes in the rate of movement required compensating video circuitry so that all segments of the characters appeared at the same brightness level upon the cathode-ray tube screen.
Still later attempts included one in which the overall character patterns were stored by storing a large number of closely spaced points along the lines of the character assemble pattern. The overall patterns were generated by generating signals connecting these small closely spaced points. Of course, large amounts of storage were required to implement these schemes because of the large number of points which necessarily had to be stored. The cost of the system is correspondingly quite typically high.